Spectrum scarcity is a problem that has been observed in regulative frequency allocation charts for some time. All potentially interesting spectrum bands for mobile communication are already allocated to services. However, additional spectrum for mobile broadband is needed to cope with the mobile broadband data exponential take-off. At the same time traditional spectrum regulatory methods are perceived too slow to adapt to the sometimes rapidly changing economic and technical requirements and may thus sometimes be seen as a burden to economical growth and innovation.
For these reasons, investigations into cognitive radio (CR) and secondary (dynamic) spectrum access have been conducted both within the academic research arena but also within the industry. The central idea behind CR, and secondary spectrum access is to use already licensed spectrum for secondary purposes, i.e., for communication between a secondary transmitter and a secondary receiver. However, the secondary usage—sometimes also referred to as white space usage—is required to somehow ensure that a primary service of the entity holding the license for the spectrum is properly protected. The secondary user may also be referred to as a white space device (WSD), which is a device that opportunistically uses spectrum licensed for a primary service on a secondary basis at times and locations where a primary user is not using the spectrum. The WSD is thus not allowed to cause harmful interference to the primary service. Furthermore, the WSD is not protected from interference from any primary service or user.
The known solutions for discovering spectrum opportunities for WSD usage, are mainly focused around three approaches:                1. Geo-location database look-ups: The WSD queries a centrally managed database referred to as the geo-location database containing information regarding channels available for secondary usage, so called white space channels. The WSD provides information regarding its location and possibly also additional information in the database query, and obtains information regarding channels available for secondary usage in the response. The channels available for secondary usage are the channels that the WSD is allowed to use for communication. Furthermore, the WSD obtains maximum allowed transmit power levels associated with the channels available for secondary usage in the response from the database. These levels are based on an estimation of how much interference that would be generated in a worst case, including a margin for aggregated interference from multiple WSD. As a database may have limitations on how often it may be updated, this approach mainly applies to cases where the primary service users are static, such as TV broadcast users. Furthermore, it is expected that WSDs will have to confirm their channel availability information at regular intervals after the initial query. The WSDs interested in using channels available for secondary usage will thus regularly make a new query to the geo-location database, in order to keep the information regarding channel availability updated.        2. Spectrum sensing: The WSD performs spectrum sensing to find out what frequencies it may use for communication, by trying to detect primary transmissions.        3. Sensing enhanced geo-location database look-ups: The geo-location database approach is enhanced by using sensing functionalities in the WSD. The sensing is used to detect mobile primary service users whose usage behavior is not known to the geo-location database, such as program making special events (PMSE) devices.        
Technical and operational requirements for operation of CR systems in the white spaces of the frequency band 470-490 MHz, have been suggested in a report from the Electronic Communications Committee (ECC) within the European Conference of Postal and Telecommunications Administrations (CEPT) (CEPT ECC SE43 “TECHNICAL AND OPERATIONAL REQUIREMENTS FOR THE POSSIBLE OPERATION OF COGNITIVE RADIO SYSTEMS IN THE ‘WHITE SPACES’ OF THE FREQUENCY BAND 470-790 MHZ”, Annex 3 to Doc. SE43(10)103). In this version of the report, a master and slave WSD configuration is considered. It is suggested that the master WSD, which may e.g. be a radio access point or more specifically a white space enabled eNB in LTE, is responsible for allocating communication resources to slave WSDs. The slave WSDs are served or controlled by the master WSD, and may e.g. be UEs in a white space enabled LTE system. The allocation of communication resources to slave WSDs should be performed in a way that ensures protection of the incumbent primary service users. However, it is not specified how this is to be performed.
In a publication regarding CR access to TV white spaces for support for home networks, it is disclosed that short range devices may communicate with an access point over white space frequencies. The access point, referred to as a home base station, manages the geo-location database access. The home base station and its associated clients are configured in a master and slave configuration. Since the home base station is connected at the end of a fixed line at a specific postcode address, its geo-location information is known. Based on the geo-location data and specific service requirements, the home base station queries the central geo-location database for channel availability through the fixed-line connection. The database returns information about various operating parameters such as number of channels, centre frequencies and associated power levels for use in that location. As short range indoor scenarios are considered, the maximum transmit power parameter made available in the database is not used. Hence, the disclosed solution is not appropriate for use over a larger service area, such as in cellular systems.
The geo-location database approach currently outlined by CEPT allows the master WSD to perform the geo-location database query. It is also possible for a master WSD to query the database for information regarding channels available for secondary usage for a whole area, possibly specified by a polygon. In case of the master-slave operation, such a possibility of querying for a whole area enables a master WSD to obtain channel availability information for the whole master WSD service area with only one query. The master WSD service area may contain a large number of slave WSDs. In case of a master WSD querying the database for its whole service area, the geo-location database would have to include a margin for aggregated interference from multiple slave WSDs when performing the calculation of permitted maximum transmit power for WSDs. As the geo-location database does not have any information of the specific slave WSD locations or of the actual usage of white space channels by the slave WSDs, the included margin would have to be based on a worst case assumption. Therefore, the allowed transmit powers for slave WSDs obtained from the geo-location database are suboptimal.